It is important to check the sights of a shotgun or rifle to be assured that when the gun is fired, the shot pattern or bullet will be directed at the same place that is visible in the sights of the gun. It has not been common practice in the past to sight in a shotgun, and usually the person using the shotgun simply relies on the original construction of the gun and sights or the preliminary setting of the sights made by the factory and does not further concern himself with the sights. It is all the more important to sight in a rifle because there is only a single projectile or bullet which must hit the mark at which the shot is being taken.
Bore sighting has traditionally been accomplished by removing the bolt of a rifle, peering through the bore of the rifle to a mark or target; and then, without moving the rifle, peering through or over the sights to the mark or another mark, making allowances for elevation and windage.
Such bore sighting is more difficult with rifles and shotguns that do not have a removable bolt, such as autoloading guns and slide action repeaters, wherein the breech is enclosed and the peering into the shell chamber is not possible except through the use of mirrors.